1. Field of the Invention
The present invention relates to a centrifugal friction clutch for an automatic transmission, including a friction clutch unit and centrifugal weights capable of engaging and disengaging the friction clutch unit according to centrifugal force acting thereon.
2. Description of the Related Art
FIG. 16 shows a prior art centrifugal friction clutch for an automatic transmission, disclosed in JP-A No. Hei 9-14436. Referring to FIG. 16, an automatic transmission 01 is provided with a planetary gear including an input shaft 02, an output shaft 03 coaxially supported for rotation on the input shaft 02, a ring gear 05, which is a helical gear, splined to the input shaft 02, a sun gear 06, which is a helical gear, connected to a one-way clutch by a rotating member 09, a carrier 08 formed integrally with the output shaft 03, and planet pinions 07, which are helical gears, supported on the carrier 08. A pressure receiving member 08a is connected to the carrier 08 formed integrally with the output shaft 03. A clutch cover 010 is supported on the pressure receiving member 08a so as to be axially movable relative to the pressure receiving member 08a and to rotate together with the pressure receiving member 08a. The clutch cover 010 has a cylindrical wall 010a surrounding the pressure receiving member 08a, an outer end wall 010b formed at one end of the cylindrical wall 010a. A pressure member 011 is fitted in the cylindrical wall 010a opposite to the pressure receiving member 08a and is retained in the cylindrical wall 010a by a snap ring 012. A plurality of friction disks 012 are interposed between the pressure receiving member 08a and the pressure member 011 and are mounted on the ring gear 05 so as to be axially movable relative to the ring gear 05 and to be rotatable together with the ring gear 05, and a plurality of clutch disks 013 are fitted in the cylindrical wall 010a of the clutch cover 010 so as to be axially movable relative to the cylindrical wall 010a and to be rotatable together with the cylindrical wall 010a. The friction disks 012 and the clutch disks 013 are arranged alternately. A plurality of rectangular openings 010c are formed in a portion of the cylindrical wall 010a of the clutch cover 010 near the outer end wall 010b in a circumferential arrangement, and centrifugal weights 015 are fitted in the rectangular openings 010c. Coned disk springs 016 and an annular bearing member 017 are interposed between the outer end wall 010b of the clutch cover 010 and the pressure receiving member 08a so that base portions 015a of the centrifugal weights 015 are held between the annular bearing member 017 and the outer end wall 010b. 
When an engine by which the automatic transmission is driven is stopped, the centrifugal weights 015 are held at positions indicated by two-dot chain lines in FIG. 16 by the coned disk springs 016 so as to engage a centrifugal friction clutch lightly, in which the pressure member 011 is pressed lightly against the pressure receiving member 08a. 
When the engine operates at a very low engine speed, the driving torque of the engine is transmitted to the planetary gear, and the pressure receiving member 08a and the pressure member 011 are spaced apart by the thrust produced by the helical gears of the planetary gear. Consequently, the friction disks 012 and the clutch disks 013 interposed between the adjacent friction disks are separated and the centrifugal friction clutch is disengaged.
When the engine operates at high engine speeds, the centrifugal weights 015 are turned radially outward to positions indicated by continuous lines in FIG. 16 by centrifugal force. Consequently, the base portions 015a of the centrifugal weights 015 presses the annular bearing member 017 to press the coned disk springs 016 against the pressure receiving member 081 which is restrained from axial movement. The resilience of the coned disk springs 016 acts through the base portions 015a of the centrifugal weights 015 on the outer end wall 010b of the clutch cover 010. Consequently, the clutch cover 010 pushes the pressure member 011 to the right, as viewed in FIG. 16 through the snap ring 012, the distance between the pressure receiving member 08a and the pressure member 011 is reduced, the friction disks 012 are pressed against the clutch disks 013 and thereby the centrifugal friction clutch is engaged.
When changing the friction disks 012 and the clutch disks 013 of the centrifugal friction clutch for maintenance, the snap ring 012 is removed, the pressure member 011 is removed, and then the friction disks 012 and the clutch disks 013 are removed. The clutch cover 010 is released from the pressure receiving member 08a when the snap ring 012 is removed. Consequently, the coned disk springs 016 are unloaded, the centrifugal weights 015 fall out of the clutch cover 010. Therefore, troublesome work for assembling the centrifugal weights 015 and the clutch cover 010 is necessary when reassembling the centrifugal friction clutch, which makes maintenance more troublesome.
The present invention has been made in view of such a problem and it is therefore an object of the present invention to provide a centrifugal friction clutch for an automatic transmission, capable of being easily disassembled and assembled for maintenance.
According to a first aspect of the present invention, a centrifugal friction clutch for an automatic transmission includes an input member; friction elements supported on the input member so as to be axially movable and to be rotated together with the input member; an output member; a clutch cover supported on the output member so as to be axially movable relative to the output member and to rotate together with the output member; clutch elements supported on the clutch cover so as to be axially slidable; centrifugal weights disposed at predetermined positions on the clutch cover; spring members holding the centrifugal weights in place; and first retaining means put in place on the clutch cover to press the friction members and the clutch members against each other so that power can be transmitted when the centrifugal weights are displaced by centrifugal force to move the clutch cover relative to the output member; wherein a first spring bearing member placed in contact with one end of each of the spring members urges the centrifugal weights toward their respective predetermined initial positions, and a second spring bearing member placed in contact with the other end of each of the spring members is held in place on the clutch cover by second retaining means engaging the clutch cover.
The spring members are held between the first and the second spring bearing member supported on the clutch cover so as to bias the centrifugal weights toward their respective predetermined initial positions, i.e., positions where the centrifugal weights are held by the spring members when the centrifugal friction clutch is assembled. Thus, the first and the second spring bearing members, the spring members and the centrifugal weights are assembled in a unit on the clutch cover. Therefore, the members assembled in a unit on the clutch cover d not come apart even if the clutch cover is disengaged from the output member by removing the first retaining means, the friction elements and the clutch elements are removed for maintenance or the like. Thus, the centrifugal friction clutch can be easily assembled and disassembled.
Preferably, the second retaining means is formed integrally with the second spring bearing member. When the second retaining means is formed integrally with the second spring bearing member, the number of component parts is reduced and assembling work is facilitated.
Preferably, the second spring bearing member is not in contact with the output member and is supported on the clutch cover such that a clearance is secured between the output member and the first retaining means. Since the spring members are not initially supported on the clutch cover and not held between the clutch cover and the output member, a space between the output member, and the first retaining means attached to the clutch cover can be optionally determined and hence the clearance can be secured between the output member and the first retaining means. Accordingly, the resilience of the spring members is not exerted on the friction elements and the clutch elements an initial state, the unnecessary frictional engagement of the friction elements and the clutch elements can be avoided, which improves the durability of the centrifugal friction clutch.